The present invention relates to optics, and more particularly, to systems that monitor the brightness of light sources.
Light-emitting diodes (LED""s) are an efficient, reliable, low-cost light source that can be used in many applications. Recent advances in LED technology have produced a variety of colors of LED""s with high luminosity per unit of required power. Such LED""s can be advantageously arranged in an array and used as a backlight for a liquid-crystal display. To save time and space, it may be possible to attach the LED""s directly on a planar circuit card or a printed wiring board.
Although LED""s have been found to be a reliable light source, in avionics and in other critical systems it may be necessary to monitor the output of the LED""s to guard against failure and to ensure constant brightness levels over time. Typically an optical feedback system is used to monitor the LED""s. An optical feedback system uses a light sensor such as a photodiode that is disposed near one or more of the LED""s in the array. If an LED fails, a change in brightness is detected by the photodiode and an appropriate signal is sent to a processor. The processor can then take appropriate action to compensate for the failed LED and can also alert a user or technician about the failure.
It has been difficult to use optical feedback techniques with LED arrays that are disposed on planar circuit boards or cards and printed wiring boards. This is because most commercially available LED""s emit light in a direction away from the circuit board to which they are attached, and most photodiodes are built to detect light that is emitted from a place other than the circuit board to which the photodiode is attached. In other words, most photodiodes are not designed to accurately detect light from an LED that is attached to the same planar substrate as the photodiode. One possible solution has been to mount the photodiode above the circuit board and point the photodiode toward the board-mounted LED. However, this solution requires a mechanical mount that is attached to the circuit board and requires hand-wiring of the photodiode to the circuitry on the circuit board. This increases the expense and time required to assemble the circuit board. Furthermore, because the electrical connection between the photodiode and the circuit board is lengthened, the photodiode has an increased sensitivity to noise.
Another challenge to effective optical feedback techniques arises when it is desired for a photodiode to monitor the brightness of more than one LED. To effectively monitor multiple LED""s, the light detected by the photodiode under normal conditions should be comprised of equal amounts of light from each LED. In practice, however, it is difficult to position the LED""s with respect to the photodiode to detect equal amount of light from each LED.
Still another problem is that contaminants such as dust and other airborne particulates between the LED""s and the photodiode can create inaccuracies in the brightness measurements by the photodiode. The effectiveness of the optical feedback is therefore compromised.
It is therefore an object of the invention to increase the effectiveness of an optical feedback system.
It is another object of the invention to provide a way to detect light from an light source using a light-detecting mechanism, wherein the light source and the light-detecting mechanism are mounted on a substantially planar substrate.
A feature of the invention is a device that reflects light from a substrate-mounted LED toward a substrate-mounted light detector.
An advantage of the invention is that the brightness of light from a light source can be easily and inexpensively monitored.
The invention provides a light reflector for use with a light source and a light detection mechanism, the light source and the light detection mechanism being mounted on a substrate. The light reflector includes a generally hollow portion. The generally hollow portion has a first end and a second end. A reflective surface is disposed upon the first end of the generally hollow portion. The second end is attached to the substrate such that the substrate and the light reflector define a substantially airtight enclosure that is of sufficient size to accommodate therein the light source and the light detection mechanism.
The invention also provides an apparatus for measuring the brightness of light. The apparatus includes a light source mounted on a planar substrate. A light detection mechanism is mounted on the planar substrate adjacent the light source. A light-reflecting component has a reflective surface disposed thereon. The reflective surface is positioned to reflect the light emitted from the light source toward the light detection mechanism. The light detection mechanism measures the brightness of the light emitted from the light source.
The invention also provides a method of detecting the brightness of light that is emitted by an LED. According to the method, the LED is mounted on a substantially planar substrate. A light detection mechanism is mounted on the substrate adjacent the LED. A light-reflecting component is mounted on the substrate such that the light-reflecting component and the substrate substantially enclose the LED and the light detection mechanism. The LED is caused to emit light, and the brightness of the light that is reflected by the light-reflecting component and thereby directed toward the light detection mechanism is measured.